It's not a graphics issue, it's an input issue which I reported during beta-testing, but which I'm not sure what the best solution would be - I simply learned to click more "definitely".
It has to do with clicking and holding just a tiny bit too long while moving the mouse. It's a problem when you try really fast to click and move to the next player's weapon and click, such as when you're trying to be super fast to get all four attacks in before the monster strikes. Due to the new system in Grimrock 2 of weapon charging, right click looks for holding the button down, and how long, so the click only registers when you lift your finger back OFF right mouse button. If your click starts on the weapon icon but ends OFF the weapon icon, the attack doesn't happen. Maybe the programming solution is to remember where the click originated and count it even if the mouse moved in the mean-time before letting go of the button.
My personal solution was to just train my finger to WHACK the mouse button more briefly.